# LED Resistor Calculator

> Calculate led resistor step by step: R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED.

Page: https://freecalculator.tech/tools/led-resistor-calculator
Last reviewed: 2026-11-03

## Formula

R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED.

- Current entered in mA is converted to amperes.
- The supply must exceed the combined forward voltage.
- Choose the next standard resistor value above the result to keep current at or below the target.
- R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED.
- Inputs used: Supply voltage, LED forward voltage, LED current, LEDs in series.

## Inputs

- Supply voltage
- LED forward voltage
- LED current
- LEDs in series

## Worked example: Red LED on 5 V at 20 mA

Single LED with Vf = 2 V.
1. Resistor voltage = 5 − 2 = 3 V.
2. R = 3 ÷ 0.02 = 150 Ω.
3. P = 3 × 0.02 = 60 mW.

## Common questions

### What if the value isn't a standard resistor?

Pick the next higher standard value. The LED will run slightly dimmer but within its current limit.

### Can I share one resistor between parallel LEDs?

It is not recommended. Forward voltages differ slightly, so one LED can hog current. Give each parallel LED its own resistor.

### Where do forward voltage and current come from?

The LED's datasheet. They vary by colour and type, so no value is assumed here.

### How do I use the LED Resistor Calculator?

Enter the required values for Supply voltage, LED forward voltage, LED current, LEDs in series. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.

### What formula does the LED Resistor Calculator use?

R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.

### Can the LED Resistor Calculator be used for exact decisions?

Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.

## Limitations and assumptions

- Forward voltage varies with temperature and batch.
- For high-power LEDs, a constant-current driver is better than a resistor.
- Supply ripple is ignored.
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this led resistor calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.

## Sources

- OpenStax (Rice University) — https://openstax.org/details/books/university-physics-volume-2

## Cite this page

- APA: Free Calculator. (2026, November 3). LED Resistor Calculator. https://freecalculator.tech/tools/led-resistor-calculator
- MLA: "LED Resistor Calculator." Free Calculator, 3 Nov. 2026, https://freecalculator.tech/tools/led-resistor-calculator.
- Link: LED Resistor Calculator — Free Calculator, https://freecalculator.tech/tools/led-resistor-calculator